Abstract Hepatitis D virus (HDV) infection is the most severe type of viral hepatitis, often causing accelerated liver damage that leads to end-stage liver disease. About 15?20 million individuals are infected by HDV worldwide. The absence of an effective treatment for acute forms of the disease and the limited efficacy of current treatments for the chronic infection justify novel strategies towards the development of anti-HDV therapeutics. The lack of HDV-encoded ?druggable? targets that are suitable for conventional therapeutic modalities such as small molecules and antibodies makes RNA interference an attractive alternative approach to target this virus. However, the highly structured, GC- rich circular genome of this smallest of RNA viruses make it a challenging target for RNA-targeting approaches such as RNAi. In Phase I of this project, we were able to identify inhibitors of HDV using SomaGenics' sshRNA (synthetic small shRNAs) platform. These sshRNAs potently inhibit viral replication at multiple target sites in a cell culture infection model. The sshRNAs were then chemically modified to improve their drug-like properties. In Phase 2, we plan to move the program forward into preclinical studies using a transgenic mouse model that supports HDV infection. We will pursue dual approaches to delivery of our therapeutic sshRNAs to liver: formulation with lipid nanoparticles and use of a targeting ligand. Patterns of chemical modification will be optimized for each candidate delivery method to permit a careful comparison, and the most promising approach will be selected. Finally, combinations of sshRNAs will be assessed for ability to provide increased efficacy and forestall the development of resistance to therapy. By the end of Phase II, we expect to have established a cocktail of inhibitors ready for IND-enabling safety studies and then commencement of clinical studies.